1. Field of the Invention
The present invention relates to a flat panel display device and more particularly, to an organic electroluminescent display (ELD) device and manufacturing method for the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been most widely used in the field of flat panel display devices due to their lightweight and low power consumption. However, the liquid crystal display (LCD) device is not a light emitting element but rather a light receiving element that needs an additional light source to display images. Thus, there is a technical limit in improving brightness, contrast ratio and viewing angle as well as in enlarging the size of a liquid crystal display panel. For these reasons, much research has been actively pursued in this field to develop a new flat panel display element that can overcome the aforementioned problems of the LCD device.
The organic electroluminescent display (ELD) device is one of the those new flat panel display elements designed to replace the LCD device. Because the organic electroluminescent display (ELD) device actually emits light, the viewing angle and the contrast ratio is superior compared to the liquid crystal display (LCD) device. In addition, because an ELD device does not need a backlight as the light source, it has advantages, such as a lightweight, small dimension and low power consumption. Moreover, the organic electroluminescent display (ELD) device can be driven with a low DC (direct current) and has a fast response time. Because the organic electroluminescent display (ELD) device uses solid material instead of fluid material, such as liquid crystal, it is more stable under external impact. Further, the organic electroluminescent display (ELD) device can be operated throughout a wider range of temperatures than the liquid crystal display (LCD) device.
The organic electroluminescent display (ELD) device also has an advantage in terms of a production cost. More specifically, a deposition apparatus and an encapsulation apparatus are all the apparatuses used for manufacturing the organic electroluminescent display (ELD) device while the liquid crystal display (LCD) device or Plasma display panels (PDPs) need many kind of apparatuses. Thus, the manufacturing process for the organic electroluminescent display (ELD) device is very simple compared to the manufacturing process for liquid crystal display (LCD) device or the Plasma display panels (PDPs).
The organic electroluminescent display (ELD) devices may be classified into a passive matrix-type and an active matrix-type. In the case of the passive matrix-type organic electroluminescent display (ELD) device, pixels are formed in a matrix between scan lines and the signal lines that cross each other. The scan lines must be sequentially driven to drive each pixel. Accordingly, an average luminance depends on the number of the scan lines. However, in the case of the active matrix-type organic electroluminescent display (ELD) device, a thin film transistor, i.e., a switching element, is formed in each sub-pixel to switch the pixel on and off. More specifically, a pixel electrode connected to the thin film transistor is turn on and off. In addition, a second electrode functions as a common electrode. Moreover, in case of the active matrix-type organic electroluminescent display (ELD) device, a voltage that is applied to the pixel is stored to a storage capacitor CST and maintained until a signal for the next frame is applied. Accordingly, the pixel can retain the signal until the next frame regardless of the number of the scan lines. Because the active matrix-type organic electroluminescent display (ELD) device can obtain the same luminance with low direct current (DC), the active matrix-type organic electroluminescent display (ELD) device has advantages of low power consumption, high resolution and a large size. A basic structure and a operational property of the active matrix-type organic electroluminescent display (ELD) device will be described hereinafter with reference to FIG. 1.
FIG. 1 is a circuit diagram of a pixel of a related art active matrix organic electroluminescent display (ELD) device. In FIG. 1, scan line 2 is formed in a first direction and signal and power supply lines 4 and 6 are formed in a second direction perpendicular to the first direction. The signal line 4 and the power supply line 6 are spaced apart from each other and define a sub-pixel by crossing the scan line 2. A switching thin film transistor 8, i.e., an addressing element, is formed at a position near an intersection of the scan and signal lines 2 and 4. A storage capacitor (CST) 12 is electrically connected to the switching thin film transistor 8 and the power supply line 6. A driving thin film transistor 10, i.e., a current source element, is electrically connected to the storage capacitor (CST) 12 and the power supply line 6 and an organic electroluminescent diode 14 is electrically connected to the driving thin film transistor 10. If current is applied to the organic light-emitting material of the organic electroluminescent display (ELD) device in a positive direction, electrons and holes recombine about P-N junction between anode electrode for proving holes and cathode electrode for proving electrons. The combined electron and the hole require less energy than the energies in both the electron and the hole when they are not combined. Thus, the organic electroluminescent display (ELD) device utilizes the principle that light is emitted as a result of the energy difference between before and after the combination of the electron and the hole.
FIG. 2 is a cross-sectional view of a related art active matrix organic electroluminescent display device. In FIG. 2, the organic electroluminescent display (ELD) device has first and second substrate 10 and 50 spaced apart from each other. An array element layer 30 including a plurality of thin film transistors formed in each sub-pixel is formed on the first substrate 10. A first electrode 32 electrically connected to the thin film transistor is formed on the array element layer 30 corresponding to each pixel. An organic light-emitting layer 34 for displaying red (R), green (G) and blue (B) colors in each sub-pixel is formed on the first electrode 32 and a second electrode 38 is formed on the organic light-emitting layer 34. The organic light-emitting layer 34 forms an organic electroluminescent diode element “E” together with the first and second electrodes 32 and 38. The second substrate 50 that is used for an encapsulation has concave portion 52 and an absorbent desiccant 54 is filled into the concave portion 52. The moisture absorbent desiccant 54 removes moisture and oxygen that may be infiltrated into an interior of the organic electroluminescent display (ELD) device. The organic electroluminescent display (ELD) device is completed by bonding the first and second substrates 10 and 50 together by disposing a sealant 70 between the first and second substrates 10 and 50.
The related art bottom emission-type organic electroluminescent display (ELD) device is commonly manufactured by forming a thin film transistor array part and an organic light-emitting part on a same substrate, and then bonding the substrate to an encapsulating structure. If the thin film transistor array part and the organic light-emitting-part are formed on the same substrate, then a yield of a panel having the thin film transistor array part and the organic light-emitting part is dependent upon the product of the individual yields of the thin film transistor array part and the organic light-emitting part. However, the yield of the panel is greatly affected by the yield of the organic light-emitting layer. Accordingly, if an inferior organic light-emitting layer that is usually formed of a thin film having a thickness of 1000 Å has a defect due to impurities and contaminants, the panel is classified as an inferior panel. This leads to wasted production costs and material, thereby decreasing the yield of the panel.
The bottom emission-type organic electroluminescent display (ELD) devices are advantageous for their high image stability and variable fabrication processing. However, the bottom emission-type organic electroluminescent display (ELD) devices are not adequate for implementation in devices that require high resolution due to limitations of increased aperture ratio. In contrast, top emission-type organic electroluminescent display (ELD) devices emit light upward of the substrate. Thus, the light can be emitted without influencing the thin film transistor array part that is positioned under the light-emitting layer. Accordingly, design of the thin film transistor may be simplified. In addition, the aperture ratio can be increased, thereby increasing operational life span of the organic electroluminescent display (ELD) device. However, since a cathode is commonly formed over the organic light-emitting layer in the top emission-type organic electroluminescent display (ELD) devices, material selection and light transmittance are limited such that light transmission efficiency is lowered. If a thin film-type passivation layer is formed to prevent a reduction of the light transmittance, the thin film passivation layer may fail to prevent infiltration of exterior air into the device.